1. Field of the Invention
The present invention is directed to a hydrostatic self-loading controlled deflection roll, and specifically to an apparatus for axially locating the shell of such a roll.
2. Description of the Prior Art
Pairs of rolls forming a nip through which a traveling web passes are used at many locations in a papermaking machine, particularly in the press section to mechanically remove water from the web. In such nips, one or both rolls are loaded, i.e., the roll is mechanically forced toward the nip in order to exert a desired amount of pressure on the web as it travels through the nip. It is also necessary to be able to mechanically retract the rolls of a nip away from each other, so as to open the nip. Such retraction is necessary not only to be able to control the nip pressure, but also as part of the start-up procedure for the papermaking machine either at the beginning of a new production run, or after a sheet break. The start-up procedure involves the cutting and threading of a "tail" through the machine at a speed which is sometimes significantly slower than the normal operating speed of the machine. However, the tail threading procedure can also be accomplished at full machine speed. During this start-up procedure, a nip will not be loaded at its normal operating pressure. For many years in the papermaking industry, loading of rolls was accomplished by suitable mechanisms disposed at one or both ends of the roll shaft about which the roll rotates. Such mechanisms moved the entire roll on its shaft toward and away from the mating roll in the nip.
In order to provide uniform processing of the entire width of the web in the cross-machine direction as it travels through a web, it is desirable to have the line of contact between the two rolls forming the nip be as straight as possible or, if one of the rolls has a contour which is not a straight line, to have the other roll follow that contour as closely as possible. As improving technology in the papermaking industry permitted papermaking machines to be made increasingly wider in the cross-machine direction, as well as to operate at increasingly faster speeds, the sheer weight of the roll or the roll shell, supported only at its opposite ends, resulted in a slight "sag" of the roll in a central region of the nip, thereby causing the line of contact between the two rolls in a nip to exhibit a non-uniform distance between the rolls along the cross-machine direction.
Controlled deflection rolls were developed in response to this problem. The first generation of such controlled deflection rolls were intentionally loaded at their opposite ends so as to cause the roll shell to exhibit a slight outward bow in opposition to the aforementioned sag, so that the distance between the two rolls in the nip would be uniform along the entire cross-machine width of the nip.
More recently, so-called self-loading controlled deflection rolls have been developed, wherein a number of hydraulically operated shoes are carried on a center shaft disposed inside the roll shell, the shoes being actuatable to move toward and away from the axis of rotation of the roll, so as to push against the inner surface of the roll shell, thereby achieving the desired deflection of the outer surface of the roll shell. The need to provide complicated mechanisms at the opposite ends of the roll to move the roll toward and away from the nip is thereby avoided, and only mechanisms for rotating the roll need to be provided at one or both ends, typically only at one end. Examples of such self-loading controlled deflection rolls are disclosed in U.S. Pat. Nos. 5,193,258, 5,127,141, 5,111,563 5,060,357 and 4,821,384.
Known hydrostatic self-loading controlled deflection rolls, such as described in the above-noted U.S. Pat. Nos. 4,821,384 and 5,060,357, make use of hydrostatic bearing pads which take the form of hydrostatic side or guide shoes. Such hydrostatic bearing pads locate the roll shell axially in a fixed location at one end of the roll, while allowing the position of the roll shell to float at the opposite end, thereby permitting differential thermal expansion between the roll center shaft and the shell to be accommodated.
The use of such hydrostatic bearing pads to axially locate the roll shell, however, requires extra pumping horsepower in order to supply these bearings with hydraulic fluid, and adds costs to the manufacture of the side shoes and the center shaft. The use of such pads also create additional chances for roll failure, because the relatively small diameter capillary tubes which throttle oil flow through the bearing pads are prone to clogging. Moreover, since the side shoes are located inside the roll shell, if maintenance is needed on the bearing pads, access to the interior of the roll shell must be gained.